Patent GB-726542-A (GB)
Title: Improvements in processes of photographic reproduction
IPC: G03C5/315
Filed: 1953-01-21
Abstract: 726,542. Photographic process. KODAK, Ltd. Jan. 21, 1953 [Jan. 21, 1952], No. 1825/53. Addition to 655,274. Class 98 (2). A method of photographic reproduction comprises the treatment of an exposed substantially unhardened light-sensitive silver salt emulsion layer containing a tanning developing agent with an alkaline solution to promote tanning development, the transference in the presence of moisture of at least a thin stratum of the unhardened parts of the emulsion layer to a support such as an absorbent support, and the treatment of the emulsion layer after exposure and before transfer to increase the optical density of the unhardened regions of the layer by forming therein silver or silver sulphide. The exposed emulsion layer may be treated during or following development with a desensitizing agent, and the desensitizing and optical density increasing treatments may be carried during the alkaline development step. The agent increasing the optical density may be incorporated in the alkaline developing solution, and the desensitizing effected in a step following development. Preferably, the desensitizing agent is incorporated in the alkaline solution used for development and the developed emulsion layer is treated with a sulphur compound, such as thiourea, to convert the unexposed silver halide to silver sulphide. In an example, a cellulose ester or paper support carries an emulsion containing the tanning developing agent 3 : 4-dihydroxydiphenyl and preferably also a pigment such as silver; carbon black, or Monastral Fast Blue BWD. The material is exposed by projection or reflex printing, is developed in an aqueous solution of sodium carbonate, urea, and potassium bromide (desensitizing agent) and excess solution is removed. The developed material is treated in an aqueous solution of sodium carbonate and thiourea to increase the optical density of the unexposed parts, excess solution is removed, and the material is pressed; emulsion side down, on to absorbent paper. On separating the sheets, the paper carries a thin stratum of the unhardened regions, and additional copies may be made by re-moistening the emulsion layer with alkaline solution and repeating the transfer. The potassium bromide may be replaced by a desensitizing dye such as Pinakyptol Green or Yellow, 2-(p-diethylaminophenyliminomethyl) - 6 - methylquinoline methochloride, 4 - nitro - 6 - chlorobenztriazole, or 6 - ethoxy- 2 - p - nitrostyrylquinoline metho - p - toluenesulphonate. The urea (gelatin softening agent) may be replaced by formamide, ethylene chlorohydrin, or sodium nitrate, and the gelatin softening agent may be applied in a separate bath following development. The thiourea may be replaced by thiosemicarbazide or sodium sulphide or a reducing solution, such as an aqueous solution of hydrazine hydrate, aminoguanidine sulphate, sodium arsenite, ferrous oxalate, or a stannite, and these agents may be added to the alkaline solution. The tanning developing agent and emulsion layer may be as described in the parent Specification, and a mixture of tanning and feebly- or non-tanning developing agents may be used as described in Specification 725,667. The emulsion binder may be gelatin, polyvinyl alcohol, polyacrylamide, or a partially hydrolysed polyvinyl ester. The exposed emulsion layer containing a tanning developing agent may be developed in an alkaline solution containing a desensitizing agent (potassium bromide) and then treated with an alkali metal sulphite solution containing an intensifying agent. Alternatively, an exposed emulsion may be developed in an alkaline solution containing potassium bromide, treated in an aqueous solution of sodium sulphite, sodium carbonate, and urea, and uniformly exposed to light to render the remaining silver halide developable and permit development of a black silver image in the unhardened parts to occur. Specification 655,273 also is referred to.